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Saturday, June 30, 2012

Class XI, BIOLOGY, "Phylum Porifera (Sponges)"

Phylum Porifera (Sponges)

MAIN CHARACTERISTICS
Animals of this phylum show following important characters.NATURE
Most simple multicellular organisms. From evolutionary point of view they occupy a position between protozoa and true metazoaHABIT AND HABITAT

Mostly marine but few in fresh water habitat.

They are sessile, living attached to rocks, coral and other hard surfaces

SHAPE AND STRUCTURE

Their shape may be cylindrical, branching, globular, flat, bell shaped or cup shaped.

Some are dull in colour and most are brightly coloured.

The body is perforated by pores and canals.

MICROSCOPIC STRUCTURE
Most of sponges contain following types of cell:(A) PINACOCYTES
Forming the epidermis.(B) POROCYTES
Form pores of the body wall(C) CHOANOCYTES
These are flagellated cells, form the internal lining of the body. These cells are strikingly similar to the choano flagellates.

Much of the body is composed of jelly like matrix containing a skeleton made of Protein, CaCO3 or silica.

Sponges are organized on cellular level, instead of a single cell carring on all the life activities.

Sponges show cellular differentiation but little or no coordination of cells to form tissues.

They usually have an endoskeleton of separate spicules.

They do not posses a head, an interior end, a mouth or gut cavity.

They are sedentary organisms ranging in size from 1 to 200cm.

DIGESTION
Digestion takes place with in the cell. (Intracellular)PROCESS OF FEEDING, EXCRETION AND RESPIRATION

Sponges feed by filtering out bacteria and fine particles of organic matter from water.

The flagella of “Choanocytes” beat and create a current of water.

The water current also helps in respiration, removal of waste products and dispersal of gametes.

REPRODUCTION

Reproduction is of both type asexual and sexual

Asexual reproduction is by means of “Buds” and “Gemmules formation”.

Sexual reproduction is by means of sperm and ova.

All sponges appear to be diploid and have the usual metazoan process of “Oogenesis” and “Spermatogenesis”.

The eggs retained just beneath the choanocytes where they are
fertilized by sperm from another sponge brought in with the current of
water.

Fertilization is internal.

LARVA

After cleavage, the larva escape from the parent to the open sea as a free swimming “Amphiblastula larva”.